1. Field of the Invention
The present invention relates to a liquid phase growth method for a silicon crystal, a manufacturing method for a solar cell and a liquid phase growth apparatus for a silicon crystal. In particular, the present invention relates to a liquid phase growth method for a silicon crystal, the method capable of achieving continuous growth and suitable for mass production, a manufacturing method for a solar cell including the same and a liquid phase growth apparatus for a silicon crystal, the apparatus used for these methods.
2. Description of the Related Art
A liquid phase growth method has the advantage that a high quality crystal having a composition close to the stoichiometric composition can be prepared because of crystal growth in a quasi-equilibrium state, and the method is thereby used as an established technology with respect to compound semiconductors, e.g., GaAs, for production of light-emitting diodes (LEDs), laser diodes and the like. Recently, liquid phase growth of Si has been attempted for the purpose of preparing a thick film, and an application to solar cells has been discussed, as disclosed in Japanese Patent Laid-Open No. 58-89874.
In generally known liquid phase growth methods, a solution containing a material targeted for growing, as a solute, is cooled to become supersaturated and, thereby, an excessive solute (a material targeted for growing) is deposited on a substrate. At that time, the solute must be dissolved into the solvent to saturation in advance of deposition (growth) of the solute on the substrate.
Common methods for dissolving a solute in a solvent include the case where a solute is mixed with a solvent while the amount of the solute is adjusted to bring about saturation at a dissolution temperature, followed by heating, and the case where heating is performed while a solvent is in contact with a large amount of (more than the amount required for saturation) base material composed of the solute, followed by keeping at a dissolution temperature to bring about saturation. In the former case, a newly-weighed solute is put into the solvent every time the growth is terminated, or the solvent is replaced with a fresh solvent including the solute. In the latter case, a base material to become the solute is put into the solvent or is pulled out of the solvent before the growth or after the growth and, finally, the base material is consumed to cause troubles in putting in or pulling out or cause shortage in the amount of dissolution. Consequently, replacement with a fresh base material is required. In all cases, apparatuses are stopped or the growth is suspended in order to replenish when raw materials are run out, so that time loss occurs. As described above, known methods have problems with respect to mass production.
Lertes (DE No. 2238205) discloses a method for directly supplying a solute from a gas phase into a solvent, in which a SiH4 gas (a raw material gas) is allowed to contact the solvent surface. However, in this case, the SiH4 gas is allowed to two-dimensionally contacts the solvent surface, Si atoms generated by decomposition at the solvent surface diffuse into the solvent to reach the surface of a growth substrate and, thereby, supply of Si is achieved. Consequently, the amount of supply of Si is not sufficient for achieving a practical growth rate.
Japanese Patent Laid-Open No. 11-292693 discloses a liquid phase growth method for a silicon crystal and a manufacturing method for a solar cell including the same. In the liquid phase growth method for a silicon crystal, a raw material gas containing at least silicon atoms is blown into a solvent, the raw material gas is decomposed while the silicon atoms are simultaneously dissolved into the above-described solvent and, thereby, the silicon atoms are supplied to the above-described solvent. Subsequently, a substrate is immersed in the solvent or is allowed to contact the solvent and, thereby, a silicon crystal is grown on the substrate. According to this proposition, the raw material can be continuously supplied in practice.
However, even in the liquid phase growth method disclosed in Japanese Patent Laid-Open No. 11-292693, the supply efficiency of the raw material and/or the dopant to the solvent must be improved in order to further increase the growth rate and to change a doping profile at will.